1. Field of the Invention
This invention relates generally to a method for fabricating complementary NPN and PNP bipolar transistors on a single substrate, and more particularly, to a method of selective molecular beam epitaxy for fabricating complementary NPN and PNP heterojunction bipolar transistors on a single substrate.
2. Discussion of the Related Art
Different methods are known in the art for fabricating desirable NPN or PNP type semiconductor devices. When fabricating circuits which include both NPN and PNP bipolar transistors it is sometimes thought to be necessary to develop the transistors on separate substrates. It is known, however, to be extremely advantageous in many applications to develop complementary NPN and PNP bipolar transistors on the same substrate. Applicability for these complementary devices include, but are not limited to, push-pull power amplifiers and circuits incorporating active loads, both known to those skilled in the art. For a more in-depth discussion of the advantages of complementary NPN/PNP bipolar transistors over separate NPN and PNP transistors, see P.R. Gray and R.G. Meyer, "Analysis and Design of Analog Integrated Circuits", (John Wiley & Sons, New York, 1977). For semiconductor devices which are fabricated from silicon it is possible to produce bipolar NPN and PNP transistors into useful circuits on the same substrate with certain existing fabrication techniques, such as diffusion and ion implantation.
In view of present day frequency and speed requirements of bipolar transistors, a much more useful device would be a heterojunction bipolar (HBT) transistor i.e., bipolar transistors which include at least one PN junction of dissimilar materials generally comprised of GaAs/AlGaAs (gallium arsenide/aluminum gallium arsenide) or InGaAs/InAlAs/InP (indium gallium arsenide/indium aluminum arsenide/indium phosphide). Fabrication techniques applicable to complementary silicon NPN and PNP bipolar transistors are generally not adequate for these types of GaAs/AlGaAs and InGaAs/InAlAs/InP HBTs.
Molecular beam epitaxy is one of the most popular methods of producing semiconductor devices requiring high precision doping and thickness constraints. Whether it be silicon homojunctions or GaAs/AlGaAs or InGaAs/InAlAs/InP heterojunction transistor devices, it has been heretofore thought impossible to fabricate NPN and PNP transistors on the same substrate using molecular beam epitaxy which result in high quality devices. One method attempted in the art has been to fabricate either an NPN or PNP profile on a substrate and then coat the just formed profile with a layer of silicon dioxide. The silicon dioxide is then removed by an appropriate method from the area of the substrate on which the opposite NPN or PNP profile was to be grown. This procedure has met with limited success in that the silicon dioxide layer has allowed damage to the already grown NPN or PNP profile during the process of growing the other profile. Consequently, such a procedure has degradated the response characteristics of the final device.
A method of using metal organic vapor phase epitaxy (MOVPE) has been recently reported in the literature to develop a complementary HBT, see David B. Slater, Jr. et al. "Monolithic Integration of Complementary HBT's By Selected MOVPE", IEEE Electron Device Letters, Vol. 11, No. 4, April 1990. In that procedure, the first PNP or NPN profile is developed using the MOVPE process and then a silicon nitride mask is deposited over the profile. The silicon nitride mask is then selectively etched away so that the remaining NPN structure can be developed. Although this procedure has met with some success, the performance of the ultimate complementary HBT device is poor. In addition, the MOVPE process does not have the same parameter control as can be accomplished by molecular beam epitaxy.
What is needed then is a molecular beam epitaxy procedure capable of growing complementary NPN and PNP HBT devices on a common substrate. It is therefore an object of the present invention to provide such a method.